1. Field
Embodiments of the present disclosure generally relate to optical systems such as laser systems. More specifically, the embodiments relate to optical systems capable of producing one or more output beams having multiple frequency-converted spectral peaks for reducing the appearance of speckle in a laser-projected image.
2. Technical Background
While blue and red semiconductor lasers are currently readily available, the progress in nitride semiconductor technology has not yet resulted in a creation of native green lasers with sufficient output power, efficiency and cost effectiveness. An attractive alternative is to use a near-infrared (1060 nm) laser diode and generate green light by frequency doubling in a nonlinear optical medium, such as periodically poled lithium niobate (PPLN) crystal. This allows for a small package size and reasonable efficiency, but results in a high level of speckle in projected images due to the high spectral and spatial coherence of the laser source.
Speckle is observed whenever a coherent light source is used to illuminate a rough surface, for example, a screen, wall, or any other object that produces a diffused reflection or transmission. Particularly, a multitude of small areas of the screen or other reflecting objects scatter light into a multitude of reflected beams with different points of origination and different propagation directions. At an observation point, for example in the eyes of an observer or at the sensor of a camera, these beams interfere constructively to form a bright spot, or destructively to form a dark spot, producing a random granular intensity pattern known as speckle. Speckle causes high spatial frequency noise in the projected image. Speckle may be characterized by grain size and contrast, the latter usually defined as a ratio of standard deviation to mean light intensity in the observation plane. For a large enough illuminated area and a small enough surface roughness, the speckle will be “fully developed,” with a brightness standard deviation of 100%. If an image is formed on the screen using a coherent light source such as a laser beam, such granular structure will represent noise resulting in serious degradation of the image quality. This noise presents a significant problem, particularly when the projector is used to display high spatial-frequency image content, such as text.
Accordingly, a need exists for optical systems that reduce the appearance of speckle to improve the image quality of laser projected images.